It is known to apply TBCs, such as yttria stabilized zirconia (YSZ) to external surfaces of air-cooled components, such as air-cooled turbine components. U.S. Pat. No. 4,405,659 to Strangman describes one such application, is assigned to the assignee of the present invention and is expressly incorporated by reference herein. In Strangman, a thin, uniform metallic bonding layer, e.g., between about 1-10 mils, is provided onto the exterior surface of a metal component, such as a turbine blade fabricated from a superalloy. The bonding layer may be a MCrAlY alloy, intermetallic aluminide or other suitable material. A relatively thinner layer of alumina, on the order of about 0.01-0.1 mil, is formed by oxidation on the bonding layer. Alternatively, the alumina layer may be formed directly on the alloy without utilizing a bond coat. The TBC, such as yttria stabilized zirconia, is then applied to the alumina layer by vapor deposition or other suitable process in the form of individual columnar segments, each of which is firmly bonded to the alumina layer of the component, but not to one another. The underlying metal and the ceramic TBC typically have different coefficients of thermal expansion. Accordingly, the gaps between the columnar segments enable thermal expansion of the underlying metal without damaging the TBC.
Over the operational life of a TBC coated component, and particularly during operation in extreme temperature environments such as are found in aircraft, the TBC exhibits failure by spalling. Failure of the TBC is related to the magnitude of the stresses associated with the inevitable growth and thickening of the alumina layer, which in turn relates to the quantity of oxygen that migrates to the interface, either through the ceramic material or via the gaps formed between the columnar grains. In aircraft applications, the rate of growth of the alumina layer is relatively slow. Consequently, TBC treated components typically tend to have reasonable service lives, although further increases are desirable.
While it is not fully understood, the relatively thin alumina layer is believed to be necessary to and responsible for the adherence of the subsequently applied TBC. It is known that the alumina layer helps protect the underlying bond coat and substrate against oxidation and corrosion, and that the TBC helps reduce corrosion of the underlying bond coat (if any) and metal by covering the bond coat and acting as a buffer between the bond coat and environmental contaminants. It is also known that the alumina layer inevitably grows and thickens over time in the presence of oxygen, and that a relatively thick alumina layer promotes spalling of the TBC. Since zirconia is relatively transparent to oxygen, a YSZ TBC does not play a major role in protecting against oxidation of the alumina layer.
As is apparent from the Strangman '659 patent, it is highly undesirable to attenuate corrosion or oxidation simply by applying a ceramic TBC so as to completely cover the bond coat, e.g., to eliminate the gaps between columnar grains. Such an arrangement would result in different rates of thermal expansion between the metal and ceramic, with rapid failure of the TBC upon thermal cycling of the component. It is similarly undesirable to initially generate a thicker alumina layer, which would promote the above-discussed earlier failure of the TBC by spalling.
However, erosion also contributes to the failure of TBCs. Aviation grade fuel is relatively free of impurities that form particulates and increase erosion of the TBC. Corrosion can also cause failure of the TBC and the underlying component.
In industrial turbine environments, in which the turbine components generally lack TBCs for economic reasons and because industrial gas turbines have historically run at lower temperatures, components routinely encounter erosion causing particulates and corrosive materials debris. This matter includes alkali rich salts such as alkali sulfates, which are indigenous to the operating environment, and products such as combustion products from burning fuel with a relatively high level of impurities. The alkali rich salts eventually condense on the exposed surfaces of the components and substantially shorten the life of the component. The fused salt corrodes alloys by dissolving the surface oxide scale. This corrosion is referred to as sulfidation attack or hot corrosion, and also leads to the above-noted failure by spalling. Failure resulting from corrosion has plagued the industrial turbine industry for many years.
One manner of protecting industrial turbine components from sulfidation attack is the generation of chromia directly to the exposed metal surface of the component, e.g., by chromize pack coating or diffusion directly into the substrate. However, chromia protection is not appropriate in relatively high temperature operating environments, such as are found in aircraft turbines. Above about 900.degree. C., chromia is unstable and reacts with available oxygen to form the volatile compound CrO3. In aircraft, air cooled turbine components routinely operate in environments above about 1350.degree. C. Accordingly, one skilled in the art would not use chromia to protect aircraft turbine components.
Where a TBC coated component has been exposed to alkali salts, it has been observed that the alkali salts will interact with the ceramic material and accelerate the growth of the alumina layer at the interface, although such action is not fully understood. To the best of our knowledge, no one has applied an overcoat to a TBC in an effort to attenuate corrosion or oxidation.
It is an object of the present invention to apply an overcoat to an article having a TBC to inhibit oxygen from migrating to the bond coat, and thereby attenuate the growth of the alumina layer and extend the service life of the article.
It is also an object of the invention to apply a corrosion-resistant overcoat to an article having a TBC.